All law enforcement, peacekeeping and military communities are often faced with the need to observe potentially hostile environments through protective shielding of some configuration. Transport vehicles such as aircraft, vehicles or vessels often suffer in adequate protection from ballistic and fragmentation threats through the windshields and surrounding windows and/or view ports. Protection of all vehicles in the combat theater has become a seriously realized requirement over the past few years due to urban deployments where typical ballistic stand-off distances are no longer afforded, and improvised explosive devices are becoming more prevalent. Some of the personal protection applications of transparent armor would be protective visors, guard shack glazings, transport vehicle, vessel and aircraft applications, currency transport vehicles, medical, communications and scientific research and development transparency applications.
The current existing transparent armor systems are typically comprised of several layers of either of the following materials: glass, polycarbonate or acrylic, adhered together by polyvinyl butylral (PVB) or urethane adhesive interlayers. The resulting armor structure can have multiple layers of the same material, or it may use a combination of any of the three depending upon the primary and any other secondary threat considerations. The four most utilized transparent ballistic and fragmentation resistant armor materials are as follows.
Bullet resistive glass laminates are relatively heavy and thick, and are manufactured with multiple layers of annealed glass, and polyvinyl butylral (PVB) interlayers from 0.015 inches to 0.120 inches thick. They can pass ballistic and fragmentation ratings and are generically not required to eliminate spall. Anti-spalling components have to be added to the existing already thick glazing, on the rear or protected side. Once attacked, the glass unit will become cracked and partial glass loss to either side will be noticed depending upon whether no spalling or low spalling is specified. Spalling is the exit of glass toward the protected or safe side during an attack, regardless of projectile penetration/access. Typical no-spalling materials utilized are polycarbonate sheet attached to the safe side of the transparent armor system, or a two part film composite manufactured from a 7 to 10 mil thick polyethylene terephthalate (PET) facing film with a 15 or 30 mil (thousandths of an inch) thick polyvinyl butylral (PVB) primary adhesive attached to the safe side of the transparent armor system.
Acrylics are lightweight synthetic materials with excellent optical clarity and a high degree of break resistance. They do offer limited ballistic resistance. However, acrylic glazing materials are highly combustible thermoplastics, and as such, support flames and emit toxic fumes when burned. They are also extremely susceptible to a loss of clarity through scratching, gouging and ultraviolet (UV) exposure. Acrylics once attacked exhibit large cracking and loss of the acrylic material especially during ballistic and heavy fragmentation attacks.
Polycarbonates are lightweight synthetic materials with many times the break resistance of equal thicknesses of plate glass with excellent optical clarity. Similar to acrylics, polycarbonates are susceptible to a lesser degree to scratching, gouging and yellowing from ultraviolet exposure. They too are not fire resistant and emit toxic fumes when burned, but are much more difficult to ignite, and generally have the capability to self-extinguish once flame is removed. Polycarbonates once attacked may show signs of slight delamination between layers directly surrounding a ballistic/fragmentation attack, and will encapsulate the bullet, depending upon composition and speed.
Glass-clad polycarbonates are composite glazings made by sandwiching a polycarbonate sheet between laminations of glass and PVB or urethanes; or by laminating a polycarbonate sheet to the backside of laminated glass as an anti-spall shield.
More recently, transparent ceramics (also referred to in some circles as glass-ceramics) have been showing significant ballistic performance capabilities at substantially reduced weights and overall thicknesses over the conventional glass/plastic systems. Polymeric material advancements, such as improvements in the optical properties of polyurethanes, have shown promise in their combined use with ceramics for further reductions in the overall weight of the finished transparent systems.
Both current and future requirements of military and peacekeeping organizations will involve regular combat actions in urban hostile environments, where single vehicles or supply convoys are at much at risk as organized troop formations. This brings with it the realization that all military and security personnel are at risk. There is also a need for reduced logistical burden in the theatre of operation, such that there is a continued strive for transportation systems that possess reduced weight and operational costs, with increased maneuverability and payload capability. This can be made possible by reducing the weight burden of the armor components. Preferably, such armor components should also not be overly thick, and should have the capability to withstand multiple repeat hits.